<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<script>
    function dianji(){
        var big= document.getElementById('dianji').style.fontSize;
        var intbig=parseInt(big)
        if(intbig<100) {
            intbig = intbig + 10;
            document.getElementById('dianji').style.fontSize=intbig+'px'
        }
        else {
            alert("字体大小是"+intbig+"不能放大了")
        }
    }
</script>
<script src="/src/test/js/jquery-3.3.1.min.js"></script>
<body>
<div > <input type="button" value="点击放大" id="dianji" onclick="dianji()" style="font-size:20px "></div>
</body>
</html>